Network Working Group W. Simpson
Request for Comments: 1331 Daydreamer
Obsoletes: RFCs 1171, 1172 May 1992
The Point-to-Point Protocol (PPP)for theTransmission of Multi-protocol Datagrams
over Point-to-Point Links
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) provides a method for transmitting
datagrams over serial point-to-point links. PPP is comprised of
three main components:
1. A method for encapsulating datagrams over serial links.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.
3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network-layer protocols.
This document defines the PPP encapsulation scheme, together with the
PPP Link Control Protocol (LCP), an extensible option negotiation
protocol which is able to negotiate a rich assortment of
configuration parameters and provides additional management
functions.
This RFC is a product of the Point-to-Point Protocol Working Group of
the Internet Engineering Task Force (IETF). Comments on this memo
should be submitted to the ietf-ppp@ucdavis.edu mailing list.
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RFC 1331 Point-to-Point Protocol May 19921. Introduction
Motivation
In the last few years, the Internet has seen explosive growth in
the number of hosts supporting TCP/IP. The vast majority of these
hosts are connected to Local Area Networks (LANs) of various
types, Ethernet being the most common. Most of the other hosts
are connected through Wide Area Networks (WANs) such as X.25 style
Public Data Networks (PDNs). Relatively few of these hosts are
connected with simple point-to-point (i.e., serial) links. Yet,
point-to-point links are among the oldest methods of data
communications and almost every host supports point-to-point
connections. For example, asynchronous RS-232-C [1] interfaces
are essentially ubiquitous.
Encapsulation
One reason for the small number of point-to-point IP links is the
lack of a standard encapsulation protocol. There are plenty of
non-standard (and at least one de facto standard) encapsulation
protocols available, but there is not one which has been agreed
upon as an Internet Standard. By contrast, standard encapsulation
schemes do exist for the transmission of datagrams over most
popular LANs.
PPP provides an encapsulation protocol over both bit-oriented
synchronous links and asynchronous links with 8 bits of data and
no parity. These links MUST be full-duplex, but MAY be either
dedicated or circuit-switched. PPP uses HDLC as a basis for the
encapsulation.
PPP has been carefully designed to retain compatibility with most
commonly used supporting hardware. In addition, an escape
mechanism is specified to allow control data such as XON/XOFF to
be transmitted transparently over the link, and to remove spurious
control data which may be injected into the link by intervening
hardware and software.
The PPP encapsulation also provides for multiplexing of different
network-layer protocols simultaneously over the same link. It is
intended that PPP provide a common solution for easy connection of
a wide variety of hosts, bridges and routers.
Some protocols expect error free transmission, and either provide
error detection only on a conditional basis, or do not provide it
at all. PPP uses the HDLC Frame Check Sequence for error
detection. This is commonly available in hardware
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RFC 1331 Point-to-Point Protocol May 1992
implementations, and a software implementation is provided.
By default, only 8 additional octets are necessary to form the
encapsulation. In environments where bandwidth is at a premium,
the encapsulation may be shortened to as few as 2 octets. To
support high speed hardware implementations, PPP provides that the
default encapsulation header and information fields fall on 32-bit
boundaries, and allows the trailer to be padded to an arbitrary
boundary.
Link Control Protocol
More importantly, the Point-to-Point Protocol defines more than
just an encapsulation scheme. In order to be sufficiently
versatile to be portable to a wide variety of environments, PPP
provides a Link Control Protocol (LCP). The LCP is used to
automatically agree upon the encapsulation format options, handle
varying limits on sizes of packets, authenticate the identity of
its peer on the link, determine when a link is functioning
properly and when it is defunct, detect a looped-back link and
other common misconfiguration errors, and terminate the link.
Network Control Protocols
Point-to-Point links tend to exacerbate many problems with the
current family of network protocols. For instance, assignment and
management of IP addresses, which is a problem even in LAN
environments, is especially difficult over circuit-switched
point-to-point links (such as dial-up modem servers). These
problems are handled by a family of Network Control Protocols
(NCPs), which each manage the specific needs required by their
respective network-layer protocols. These NCPs are defined in
other documents.
Configuration
It is intended that PPP be easy to configure. By design, the
standard defaults should handle all common configurations. The
implementor may specify improvements to the default configuration,
which are automatically communicated to the peer without operator
intervention. Finally, the operator may explicitly configure
options for the link which enable the link to operate in
environments where it would otherwise be impossible.
This self-configuration is implemented through an extensible
option negotiation mechanism, wherein each end of the link
describes to the other its capabilities and requirements.
Although the option negotiation mechanism described in this
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RFC 1331 Point-to-Point Protocol May 1992
document is specified in terms of the Link Control Protocol (LCP),
the same facilities may be used by the Internet Protocol Control
Protocol (IPCP) and others in the family of NCPs.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST
This word, or the adjective "required", means that the definition
is an absolute requirement of the specification.
MUST NOT
This phrase means that the definition is an absolute prohibition
of the specification.
SHOULD
This word, or the adjective "recommended", means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and carefully
weighed before choosing a different course.
MAY
This word, or the adjective "optional", means that this item is
one of an allowed set of alternatives. An implementation which
does not include this option MUST be prepared to interoperate with
another implementation which does include the option.
1.2. Terminology
This document frequently uses the following terms:
peer
The other end of the point-to-point link.
silently discard
This means the implementation discards the packet without further
processing. The implementation SHOULD provide the capability of
logging the error, including the contents of the silently
discarded packet, and SHOULD record the event in a statistics
counter.
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RFC 1331 Point-to-Point Protocol May 19922. Physical Layer Requirements
The Point-to-Point Protocol is capable of operating across any
DTE/DCE interface (e.g., EIA RS-232-C, EIA RS-422, EIA RS-423 and
CCITT V.35). The only absolute requirement imposed by PPP is the
provision of a full-duplex circuit, either dedicated or circuit-
switched, which can operate in either an asynchronous (start/stop) or
synchronous bit-serial mode, transparent to PPP Data Link Layer
frames. PPP does not impose any restrictions regarding transmission
rate, other than those imposed by the particular DTE/DCE interface in
use.
PPP does not require any particular synchronous encoding, such as FM,
NRZ, or NRZI.
Implementation Note:
NRZ is currently most widely available, and on that basis is
recommended as a default. When configuration of the encoding is
allowed, NRZI is recommended as an alternative, because of its
relative immunity to signal inversion configuration errors.
PPP does not require the use of modem control signals, such as
Request To Send (RTS), Clear To Send (CTS), Data Carrier Detect
(DCD), and Data Terminal Ready (DTR).
Implementation Note:
When available, using such signals can allow greater functionality
and performance. In particular, such signals SHOULD be used to
signal the Up and Down events in the Option Negotiation Automaton
(described below).
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RFC 1331 Point-to-Point Protocol May 19923. The Data Link Layer
The Point-to-Point Protocol uses the principles, terminology, and
frame structure of the International Organization For
Standardization's (ISO) High-level Data Link Control (HDLC)
procedures (ISO 3309-1979 [2]), as modified by ISO 3309:1984/PDAD1
"Addendum 1: Start/stop transmission" [5]. ISO 3309-1979 specifies
the HDLC frame structure for use in synchronous environments. ISO
3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to
allow its use in asynchronous environments.
The PPP control procedures use the definitions and Control field
encodings standardized in ISO 4335-1979 [3] and ISO 4335-
1979/Addendum 1-1979 [4]. The PPP frame structure is also consistent
with CCITT Recommendation X.25 LAPB [6], since that too is based on
HDLC.
The purpose of this memo is not to document what is already
standardized in ISO 3309. We assume that the reader is already
familiar with HDLC, or has access to a copy of [2] or [6]. Instead,
this paper attempts to give a concise summary and point out specific
options and features used by PPP. Since "Addendum 1: Start/stop
transmission", is not yet standardized and widely available, it is
summarized in Appendix A.
To remain consistent with standard Internet practice, and avoid
confusion for people used to reading RFCs, all binary numbers in the
following descriptions are in Most Significant Bit to Least
Significant Bit order, reading from left to right, unless otherwise
indicated. Note that this is contrary to standard ISO and CCITT
practice which orders bits as transmitted (i.e., network bit order).
Keep this in mind when comparing this document with the international
standards documents.
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RFC 1331 Point-to-Point Protocol May 19923.1. Frame Format
A summary of the standard PPP frame structure is shown below. This
figure does not include start/stop bits (for asynchronous links), nor
any bits or octets inserted for transparency. The fields are
transmitted from left to right.
+----------+----------+----------+----------+------------
| Flag | Address | Control | Protocol | Information
| 01111110 | 11111111 | 00000011 | 16 bits | *
+----------+----------+----------+----------+------------
---+----------+----------+-----------------
| FCS | Flag | Inter-frame Fill
| 16 bits | 01111110 | or next Address
---+----------+----------+-----------------
Inter-frame Time Fill
For asynchronous links, inter-frame time fill SHOULD be accomplished
in the same manner as inter-octet time fill, by transmitting
continuous "1" bits (mark-hold state).
For synchronous links, the Flag Sequence SHOULD be transmitted during
inter-frame time fill. There is no provision for inter-octet time
fill.
Implementation Note:
Mark idle (continuous ones) SHOULD NOT be used for idle
synchronous inter-frame time fill. However, certain types of
circuit-switched links require the use of mark idle, particularly
those that calculate accounting based on bit activity. When mark
idle is used on a synchronous link, the implementation MUST ensure
at least 15 consecutive "1" bits between Flags, and that the Flag
Sequence is generated at the beginning and end of a frame.
Flag Sequence
The Flag Sequence is a single octet and indicates the beginning or
end of a frame. The Flag Sequence consists of the binary sequence
01111110 (hexadecimal 0x7e).
The Flag is a frame separator. Only one Flag is required between two
frames. Two consecutive Flags constitute an empty frame, which is
ignored.
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RFC 1331 Point-to-Point Protocol May 1992
Implementation Note:
The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT
be used. When not avoidable, such an implementation MUST ensure
that the first Flag Sequence detected (the end of the frame) is
promptly communicated to the link layer.
Address Field
The Address field is a single octet and contains the binary sequence
11111111 (hexadecimal 0xff), the All-Stations address. PPP does not
assign individual station addresses. The All-Stations address MUST
always be recognized and received. The use of other address lengths
and values may be defined at a later time, or by prior agreement.
Frames with unrecognized Addresses SHOULD be silently discarded, and
reported through the normal network management facility.
Control Field
The Control field is a single octet and contains the binary sequence
00000011 (hexadecimal 0x03), the Unnumbered Information (UI) command
with the P/F bit set to zero. Frames with other Control field values
SHOULD be silently discarded.
Protocol Field
The Protocol field is two octets and its value identifies the
protocol encapsulated in the Information field of the frame.
This Protocol field is defined by PPP and is not a field defined by
HDLC. However, the Protocol field is consistent with the ISO 3309
extension mechanism for Address fields. All Protocols MUST be odd;
the least significant bit of the least significant octet MUST equal
"1". Also, all Protocols MUST be assigned such that the least
significant bit of the most significant octet equals "0". Frames
received which don't comply with these rules MUST be considered as
having an unrecognized Protocol, and handled as specified by the LCP.
The Protocol field is transmitted and received most significant octet
first.
Protocol field values in the "0---" to "3---" range identify the
network-layer protocol of specific datagrams, and values in the "8--
-" to "b---" range identify datagrams belonging to the associated
Network Control Protocols (NCPs), if any.
Protocol field values in the "4---" to "7---" range are used for
protocols with low volume traffic which have no associated NCP.
Protocol field values in the "c---" to "f---" range identify
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RFC 1331 Point-to-Point Protocol May 1992
Information Field
The Information field is zero or more octets. The Information field
contains the datagram for the protocol specified in the Protocol
field. The end of the Information field is found by locating the
closing Flag Sequence and allowing two octets for the Frame Check
Sequence field. The default maximum length of the Information field
is 1500 octets. By negotiation, consenting PPP implementations may
use other values for the maximum Information field length.
On transmission, the Information field may be padded with an
arbitrary number of octets up to the maximum length. It is the
responsibility of each protocol to disambiguate padding octets from
real information.
Frame Check Sequence (FCS) Field
The Frame Check Sequence field is normally 16 bits (two octets). The
use of other FCS lengths may be defined at a later time, or by prior
agreement.
The FCS field is calculated over all bits of the Address, Control,
Protocol and Information fields not including any start and stop bits
(asynchronous) and any bits (synchronous) or octets (asynchronous)
inserted for transparency. This does not include the Flag Sequences
or the FCS field itself. The FCS is transmitted with the coefficient
of the highest term first.
Note: When octets are received which are flagged in the Async-
Control-Character-Map, they are discarded before calculating the
FCS. See the description in Appendix A.
For more information on the specification of the FCS, see ISO 3309
[2] or CCITT X.25 [6].
Note: A fast, table-driven implementation of the 16-bit FCS
algorithm is shown in Appendix B. This implementation is based on
[7], [8], and [9].
Modifications to the Basic Frame Format
The Link Control Protocol can negotiate modifications to the standard
PPP frame structure. However, modified frames will always be clearly
distinguishable from standard frames.
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RFC 1331 Point-to-Point Protocol May 19924. PPP Link Operation4.1. Overview
In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established, the peer may be
authenticated. Then, PPP must send NCP packets to choose and
configure one or more network-layer protocols. Once each of the
chosen network-layer protocols has been configured, datagrams from
each network-layer protocol can be sent over the link.
The link will remain configured for communications until explicit LCP
or NCP packets close the link down, or until some external event
occurs (an inactivity timer expires or network administrator
intervention).
4.2. Phase Diagram
In the process of configuring, maintaining and terminating the
point-to-point link, the PPP link goes through several distinct
phases:
+------+ +-----------+ +--------------+
| | UP | | OPENED | | SUCCESS/NONE
| Dead |------->| Establish |---------->| Authenticate |--+
| | | | | | |
+------+ +-----------+ +--------------+ |
^ FAIL | FAIL | |
+<--------------+ +----------+ |
| | |
| +-----------+ | +---------+ |
| DOWN | | | CLOSING | | |
+------------| Terminate |<---+<----------| Network |<-+
| | | |
+-----------+ +---------+
4.3. Link Dead (physical-layer not ready)
The link necessarily begins and ends with this phase. When an
external event (such as carrier detection or network administrator
configuration) indicates that the physical-layer is ready to be used,
PPP will proceed to the Link Establishment phase.
During this phase, the LCP automaton (described below) will be in the
Initial or Starting states. The transition to the Link Establishment
phase will signal an Up event to the automaton.
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RFC 1331 Point-to-Point Protocol May 1992
Implementation Note:
Typically, a link will return to this phase automatically after
the disconnection of a modem. In the case of a hard-wired line,
this phase may be extremely short -- merely long enough to detect
the presence of the device.
4.4. Link Establishment Phase
The Link Control Protocol (LCP) is used to establish the connection
through an exchange of Configure packets. This exchange is complete,
and the LCP Opened state entered, once a Configure-Ack packet
(described below) has been both sent and received. Any non-LCP
packets received during this phase MUST be silently discarded.
All Configuration Options are assumed to be at default values unless
altered by the configuration exchange. See the section on LCP
Configuration Options for further discussion.
It is important to note that only Configuration Options which are
independent of particular network-layer protocols are configured by
LCP. Configuration of individual network-layer protocols is handled
by separate Network Control Protocols (NCPs) during the Network-Layer
Protocol phase.
4.5. Authentication Phase
On some links it may be desirable to require a peer to authenticate
itself before allowing network-layer protocol packets to be
exchanged.
By default, authentication is not necessary. If an implementation
requires that the peer authenticate with some specific authentication
protocol, then it MUST negotiate the use of that authentication
protocol during Link Establishment phase.
Authentication SHOULD take place as soon as possible after link
establishment. However, link quality determination MAY occur
concurrently. An implementation MUST NOT allow the exchange of link
quality determination packets to delay authentication indefinitely.
Advancement from the Authentication phase to the Network-Layer
Protocol phase MUST NOT occur until the peer is successfully
authenticated using the negotiated authentication protocol. In the
event of failure to authenticate, PPP SHOULD proceed instead to the
Link Termination phase.
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RFC 1331 Point-to-Point Protocol May 19924.6. Network-Layer Protocol Phase
Once PPP has finished the previous phases, each network-layer
protocol (such as IP) MUST be separately configured by the
appropriate Network Control Protocol (NCP).
Each NCP may be Opened and Closed at any time.
Implementation Note:
Because an implementation may initially use a significant amount
of time for link quality determination, implementations SHOULD
avoid fixed timeouts when waiting for their peers to configure a
NCP.
After a NCP has reached the Opened state, PPP will carry the
corresponding network-layer protocol packets. Any network-layer
protocol packets received when the corresponding NCP is not in the
Opened state SHOULD be silently discarded.
During this phase, link traffic consists of any possible combinations
of LCP, NCP, and network-layer protocol packets. Any NCP or
network-layer protocol packets received during any other phase SHOULD
be silently discarded.
Implementation Note:
There is an exception to the preceding paragraphs, due to the
availability of the LCP Protocol-Reject (described below). While
LCP is in the Opened state, any protocol packet which is
unsupported by the implementation MUST be returned in a Protocol-
Reject. Only supported protocols are silently discarded.
4.7. Link Termination Phase
PPP may terminate the link at any time. This will usually be done at
the request of a human user, but might happen because of a physical
event such as the loss of carrier, authentication failure, link
quality failure, or the expiration of an idle-period timer.
LCP is used to close the link through an exchange of Terminate
packets. When the link is closing, PPP informs the network-layer
protocols so that they may take appropriate action.
After the exchange of Terminate packets, the implementation SHOULD
signal the physical-layer to disconnect in order to enforce the
termination of the link, particularly in the case of an
authentication failure. The sender of the Terminate-Request SHOULD
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RFC 1331 Point-to-Point Protocol May 1992
disconnect after receiving a Terminate-Ack, or after the Restart
counter expires. The receiver of a Terminate-Request SHOULD wait for
the peer to disconnect, and MUST NOT disconnect until at least one
Restart time has passed after sending a Terminate-Ack. PPP SHOULD
proceed to the Link Dead phase.
Implementation Note:
The closing of the link by LCP is sufficient. There is no need
for each NCP to send a flurry of Terminate packets. Conversely,
the fact that a NCP has Closed is not sufficient reason to cause
the termination of the PPP link, even if that NCP was the only
currently NCP in the Opened state.
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RFC 1331 Point-to-Point Protocol May 19925.3. States
Following is a more detailed description of each automaton state.
Initial
In the Initial state, the lower layer is unavailable (Down), and
no Open has occurred. The Restart timer is not running in the
Initial state.
Starting
The Starting state is the Open counterpart to the Initial state.
An administrative Open has been initiated, but the lower layer is
still unavailable (Down). The Restart timer is not running in the
Starting state.
When the lower layer becomes available (Up), a Configure-Request
is sent.
Closed
In the Closed state, the link is available (Up), but no Open has
occurred. The Restart timer is not running in the Closed state.
Upon reception of Configure-Request packets, a Terminate-Ack is
sent. Terminate-Acks are silently discarded to avoid creating a
loop.
Stopped
The Stopped state is the Open counterpart to the Closed state. It
is entered when the automaton is waiting for a Down event after
the This-Layer-Finished action, or after sending a Terminate-Ack.
The Restart timer is not running in the Stopped state.
Upon reception of Configure-Request packets, an appropriate
response is sent. Upon reception of other packets, a Terminate-
Ack is sent. Terminate-Acks are silently discarded to avoid
creating a loop.
Rationale:
The Stopped state is a junction state for link termination,
link configuration failure, and other automaton failure modes.
These potentially separate states have been combined.
There is a race condition between the Down event response (from
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RFC 1331 Point-to-Point Protocol May 1992
the This-Layer-Finished action) and the Receive-Configure-
Request event. When a Configure-Request arrives before the
Down event, the Down event will supercede by returning the
automaton to the Starting state. This prevents attack by
repetition.
Implementation Option:
After the peer fails to respond to Configure-Requests, an
implementation MAY wait passively for the peer to send
Configure-Requests. In this case, the This-Layer-Finished
action is not used for the TO- event in states Req-Sent, Ack-
Rcvd and Ack-Sent.
This option is useful for dedicated circuits, or circuits which
have no status signals available, but SHOULD NOT be used for
switched circuits.
Closing
In the Closing state, an attempt is made to terminate the
connection. A Terminate-Request has been sent and the Restart
timer is running, but a Terminate-Ack has not yet been received.
Upon reception of a Terminate-Ack, the Closed state is entered.
Upon the expiration of the Restart timer, a new Terminate-Request
is transmitted and the Restart timer is restarted. After the
Restart timer has expired Max-Terminate times, this action may be
skipped, and the Closed state may be entered.
Stopping
The Stopping state is the Open counterpart to the Closing state.
A Terminate-Request has been sent and the Restart timer is
running, but a Terminate-Ack has not yet been received.
Rationale:
The Stopping state provides a well defined opportunity to
terminate a link before allowing new traffic. After the link
has terminated, a new configuration may occur via the Stopped
or Starting states.
Request-Sent
In the Request-Sent state an attempt is made to configure the
connection. A Configure-Request has been sent and the Restart
timer is running, but a Configure-Ack has not yet been received
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RFC 1331 Point-to-Point Protocol May 1992
nor has one been sent.
Ack-Received
In the Ack-Received state, a Configure-Request has been sent and a
Configure-Ack has been received. The Restart timer is still
running since a Configure-Ack has not yet been sent.
Ack-Sent
In the Ack-Sent state, a Configure-Request and a Configure-Ack
have both been sent but a Configure-Ack has not yet been received.
The Restart timer is always running in the Ack-Sent state.
Opened
In the Opened state, a Configure-Ack has been both sent and
received. The Restart timer is not running in the Opened state.
When entering the Opened state, the implementation SHOULD signal
the upper layers that it is now Up. Conversely, when leaving the
Opened state, the implementation SHOULD signal the upper layers
that it is now Down.
5.4. Events
Transitions and actions in the automaton are caused by events.
Up
The Up event occurs when a lower layer indicates that it is ready
to carry packets. Typically, this event is used to signal LCP
that the link is entering Link Establishment phase, or used to
signal a NCP that the link is entering Network-Layer Protocol
phase.
Down
The Down event occurs when a lower layer indicates that it is no
longer ready to carry packets. Typically, this event is used to
signal LCP that the link is entering Link Dead phase, or used to
signal a NCP that the link is leaving Network-Layer Protocol
phase.
Open
The Open event indicates that the link is administratively
available for traffic; that is, the network administrator (human
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RFC 1331 Point-to-Point Protocol May 1992
or program) has indicated that the link is allowed to be Opened.
When this event occurs, and the link is not in the Opened state,
the automaton attempts to send configuration packets to the peer.
If the automaton is not able to begin configuration (the lower
layer is Down, or a previous Close event has not completed), the
establishment of the link is automatically delayed.
When a Terminate-Request is received, or other events occur which
cause the link to become unavailable, the automaton will progress
to a state where the link is ready to re-open. No additional
administrative intervention should be necessary.
Implementation Note:
Experience has shown that users will execute an additional Open
command when they want to renegotiate the link. Since this is
not the meaning of the Open event, it is suggested that when an
Open user command is executed in the Opened, Closing, Stopping,
or Stopped states, the implementation issue a Down event,
immediately followed by an Up event. This will cause the
renegotiation of the link, without any harmful side effects.
Close
The Close event indicates that the link is not available for
traffic; that is, the network administrator (human or program) has
indicated that the link is not allowed to be Opened. When this
event occurs, and the link is not in the Closed state, the
automaton attempts to terminate the connection. Futher attempts
to re-configure the link are denied until a new Open event occurs.
Timeout (TO+,TO-)
This event indicates the expiration of the Restart timer. The
Restart timer is used to time responses to Configure-Request and
Terminate-Request packets.
The TO+ event indicates that the Restart counter continues to be
greater than zero, which triggers the corresponding Configure-
Request or Terminate-Request packet to be retransmitted.
The TO- event indicates that the Restart counter is not greater
than zero, and no more packets need to be retransmitted.
Receive-Configure-Request (RCR+,RCR-)
This event occurs when a Configure-Request packet is received from
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RFC 1331 Point-to-Point Protocol May 1992
the peer. The Configure-Request packet indicates the desire to
open a connection and may specify Configuration Options. The
Configure-Request packet is more fully described in a later
section.
The RCR+ event indicates that the Configure-Request was
acceptable, and triggers the transmission of a corresponding
Configure-Ack.
The RCR- event indicates that the Configure-Request was
unacceptable, and triggers the transmission of a corresponding
Configure-Nak or Configure-Reject.
Implementation Note:
These events may occur on a connection which is already in the
Opened state. The implementation MUST be prepared to
immediately renegotiate the Configuration Options.
Receive-Configure-Ack (RCA)
The Receive-Configure-Ack event occurs when a valid Configure-Ack
packet is received from the peer. The Configure-Ack packet is a
positive response to a Configure-Request packet. An out of
sequence or otherwise invalid packet is silently discarded.
Implementation Note:
Since the correct packet has already been received before
reaching the Ack-Rcvd or Opened states, it is extremely
unlikely that another such packet will arrive. As specified,
all invalid Ack/Nak/Rej packets are silently discarded, and do
not affect the transitions of the automaton.
However, it is not impossible that a correctly formed packet
will arrive through a coincidentally-timed cross-connection.
It is more likely to be the result of an implementation error.
At the very least, this occurance should be logged.
Receive-Configure-Nak/Rej (RCN)
This event occurs when a valid Configure-Nak or Configure-Reject
packet is received from the peer. The Configure-Nak and
Configure-Reject packets are negative responses to a Configure-
Request packet. An out of sequence or otherwise invalid packet is
silently discarded.
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RFC 1331 Point-to-Point Protocol May 1992
Implementation Note:
Although the Configure-Nak and Configure-Reject cause the same
state transition in the automaton, these packets have
significantly different effects on the Configuration Options
sent in the resulting Configure-Request packet.
Receive-Terminate-Request (RTR)
The Receive-Terminate-Request event occurs when a Terminate-
Request packet is received. The Terminate-Request packet
indicates the desire of the peer to close the connection.
Implementation Note:
This event is not identical to the Close event (see above), and
does not override the Open commands of the local network
administrator. The implementation MUST be prepared to receive
a new Configure-Request without network administrator
intervention.
Receive-Terminate-Ack (RTA)
The Receive-Terminate-Ack event occurs when a Terminate-Ack packet
is received from the peer. The Terminate-Ack packet is usually a
response to a Terminate-Request packet. The Terminate-Ack packet
may also indicate that the peer is in Closed or Stopped states,
and serves to re-synchronize the link configuration.
Receive-Unknown-Code (RUC)
The Receive-Unknown-Code event occurs when an un-interpretable
packet is received from the peer. A Code-Reject packet is sent in
response.
Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
This event occurs when a Code-Reject or a Protocol-Reject packet
is received from the peer.
The RXJ+ event arises when the rejected value is acceptable, such
as a Code-Reject of an extended code, or a Protocol-Reject of a
NCP. These are within the scope of normal operation. The
implementation MUST stop sending the offending packet type.
The RXJ- event arises when the rejected value is catastrophic,
such as a Code-Reject of Configure-Request, or a Protocol-Reject
of LCP! This event communicates an unrecoverable error that
Simpson [Page 23]

RFC 1331 Point-to-Point Protocol May 1992
terminates the connection.
Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
(RXR)
This event occurs when an Echo-Request, Echo-Reply or Discard-
Request packet is received from the peer. The Echo-Reply packet
is a response to a Echo-Request packet. There is no reply to an
Echo-Reply or Discard-Request packet.
5.5. Actions
Actions in the automaton are caused by events and typically indicate
the transmission of packets and/or the starting or stopping of the
Restart timer.
Illegal-Event (-)
This indicates an event that SHOULD NOT occur. The implementation
probably has an internal error.
This-Layer-Up (tlu)
This action indicates to the upper layers that the automaton is
entering the Opened state.
Typically, this action MAY be used by the LCP to signal the Up
event to a NCP, Authentication Protocol, or Link Quality Protocol,
or MAY be used by a NCP to indicate that the link is available for
its traffic.
This-Layer-Down (tld)
This action indicates to the upper layers that the automaton is
leaving the Opened state.
Typically, this action MAY be used by the LCP to signal the Down
event to a NCP, Authentication Protocol, or Link Quality Protocol,
or MAY be used by a NCP to indicate that the link is no longer
available for its traffic.
This-Layer-Start (tls)
This action indicates to the lower layers that the automaton is
entering the Starting state, and the lower layer is needed for the
link. The lower layer SHOULD respond with an Up event when the
lower layer is available.
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RFC 1331 Point-to-Point Protocol May 1992
This action is highly implementation dependent.
This-Layer-Finished (tlf)
This action indicates to the lower layers that the automaton is
entering the Stopped or Closed states, and the lower layer is no
longer needed for the link. The lower layer SHOULD respond with a
Down event when the lower layer has terminated.
Typically, this action MAY be used by the LCP to advance to the
Link Dead phase, or MAY be used by a NCP to indicate to the LCP
that the link may terminate when there are no other NCPs open.
This action is highly implementation dependent.
Initialize-Restart-Counter (irc)
This action sets the Restart counter to the appropriate value
(Max-Terminate or Max-Configure). The counter is decremented for
each transmission, including the first.
Zero-Restart-Counter (zrc)
This action sets the Restart counter to zero.
Implementation Note:
This action enables the FSA to pause before proceeding to the
desired final state. In addition to zeroing the Restart
counter, the implementation MUST set the timeout period to an
appropriate value.
Send-Configure-Request (scr)
The Send-Configure-Request action transmits a Configure-Request
packet. This indicates the desire to open a connection with a
specified set of Configuration Options. The Restart timer is
started when the Configure-Request packet is transmitted, to guard
against packet loss. The Restart counter is decremented each time
a Configure-Request is sent.
Send-Configure-Ack (sca)
The Send-Configure-Ack action transmits a Configure-Ack packet.
This acknowledges the reception of a Configure-Request packet with
an acceptable set of Configuration Options.
Simpson [Page 25]

RFC 1331 Point-to-Point Protocol May 1992
Send-Configure-Nak (scn)
The Send-Configure-Nak action transmits a Configure-Nak or
Configure-Reject packet, as appropriate. This negative response
reports the reception of a Configure-Request packet with an
unacceptable set of Configuration Options. Configure-Nak packets
are used to refuse a Configuration Option value, and to suggest a
new, acceptable value. Configure-Reject packets are used to
refuse all negotiation about a Configuration Option, typically
because it is not recognized or implemented. The use of
Configure-Nak versus Configure-Reject is more fully described in
the section on LCP Packet Formats.
Send-Terminate-Request (str)
The Send-Terminate-Request action transmits a Terminate-Request
packet. This indicates the desire to close a connection. The
Restart timer is started when the Terminate-Request packet is
transmitted, to guard against packet loss. The Restart counter is
decremented each time a Terminate-Request is sent.
Send-Terminate-Ack (sta)
The Send-Terminate-Ack action transmits a Terminate-Ack packet.
This acknowledges the reception of a Terminate-Request packet or
otherwise serves to synchronize the state machines.
Send-Code-Reject (scj)
The Send-Code-Reject action transmits a Code-Reject packet. This
indicates the reception of an unknown type of packet.
Send-Echo-Reply (ser)
The Send-Echo-Reply action transmits an Echo-Reply packet. This
acknowledges the reception of an Echo-Request packet.
5.6. Loop Avoidance
The protocol makes a reasonable attempt at avoiding Configuration
Option negotiation loops. However, the protocol does NOT guarantee
that loops will not happen. As with any negotiation, it is possible
to configure two PPP implementations with conflicting policies that
will never converge. It is also possible to configure policies which
do converge, but which take significant time to do so. Implementors
should keep this in mind and should implement loop detection
mechanisms or higher level timeouts.
Simpson [Page 26]

RFC 1331 Point-to-Point Protocol May 19925.7. Counters and Timers
Restart Timer
There is one special timer used by the automaton. The Restart timer
is used to time transmissions of Configure-Request and Terminate-
Request packets. Expiration of the Restart timer causes a Timeout
event, and retransmission of the corresponding Configure-Request or
Terminate-Request packet. The Restart timer MUST be configurable,
but MAY default to three (3) seconds.
Implementation Note:
The Restart timer SHOULD be based on the speed of the link. The
default value is designed for low speed (19,200 bps or less), high
switching latency links (typical telephone lines). Higher speed
links, or links with low switching latency, SHOULD have
correspondingly faster retransmission times.
Max-Terminate
There is one required restart counter for Terminate-Requests. Max-
Terminate indicates the number of Terminate-Request packets sent
without receiving a Terminate-Ack before assuming that the peer is
unable to respond. Max-Terminate MUST be configurable, but should
default to two (2) transmissions.
Max-Configure
A similar counter is recommended for Configure-Requests. Max-
Configure indicates the number of Configure-Request packets sent
without receiving a valid Configure-Ack, Configure-Nak or Configure-
Reject before assuming that the peer is unable to respond. Max-
Configure MUST be configurable, but should default to ten (10)
transmissions.
Max-Failure
A related counter is recommended for Configure-Nak. Max-Failure
indicates the number of Configure-Nak packets sent without sending a
Configure-Ack before assuming that configuration is not converging.
Any further Configure-Nak packets are converted to Configure-Reject
packets. Max-Failure MUST be configurable, but should default to ten
(10) transmissions.
Simpson [Page 27]

RFC 1331 Point-to-Point Protocol May 19926. LCP Packet Formats
There are three classes of LCP packets:
1. Link Configuration packets used to establish and configure a
link (Configure-Request, Configure-Ack, Configure-Nak and
Configure-Reject).
2. Link Termination packets used to terminate a link (Terminate-
Request and Terminate-Ack).
3. Link Maintenance packets used to manage and debug a link
(Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
Discard-Request).
This document describes Version 1 of the Link Control Protocol. In
the interest of simplicity, there is no version field in the LCP
packet. If a new version of LCP is necessary in the future, the
intention is that a new Data Link Layer Protocol field value will be
used to differentiate Version 1 LCP from all other versions. A
correctly functioning Version 1 LCP implementation will always
respond to unknown Protocols (including other versions) with an
easily recognizable Version 1 packet, thus providing a deterministic
fallback mechanism for implementations of other versions.
Regardless of which Configuration Options are enabled, all LCP Link
Configuration, Link Termination, and Code-Reject packets (codes 1
through 7) are always sent in the full, standard form, as if no
Configuration Options were enabled. This ensures that LCP
Configure-Request packets are always recognizable even when one end
of the link mistakenly believes the link to be open.
Exactly one Link Control Protocol packet is encapsulated in the
Information field of PPP Data Link Layer frames where the Protocol
field indicates type hex c021 (Link Control Protocol).
A summary of the Link Control Protocol packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
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RFC 1331 Point-to-Point Protocol May 1992
Code
The Code field is one octet and identifies the kind of LCP packet.
When a packet is received with an invalid Code field, a Code-
Reject packet is transmitted.
The most up-to-date values of the LCP Code field are specified in
the most recent "Assigned Numbers" RFC [11]. Current values are
assigned as follows:
1 Configure-Request
2 Configure-Ack
3 Configure-Nak
4 Configure-Reject
5 Terminate-Request
6 Terminate-Ack
7 Code-Reject
8 Protocol-Reject
9 Echo-Request
10 Echo-Reply
11 Discard-Request
12 RESERVED
Identifier
The Identifier field is one octet and aids in matching requests
and replies. When a packet is received with an invalid Identifier
field, the packet is silently discarded.
Length
The Length field is two octets and indicates the length of the LCP
packet including the Code, Identifier, Length and Data fields.
Octets outside the range of the Length field should be treated as
Data Link Layer padding and should be ignored on reception. When
a packet is received with an invalid Length field, the packet is
silently discarded.
Data
The Data field is zero or more octets as indicated by the Length
field. The format of the Data field is determined by the Code
field.
Simpson [Page 29]

RFC 1331 Point-to-Point Protocol May 19926.1. Configure-Request
Description
A LCP implementation wishing to open a connection MUST transmit a
LCP packet with the Code field set to 1 (Configure-Request) and
the Options field filled with any desired changes to the default
link Configuration Options.
Upon reception of a Configure-Request, an appropriate reply MUST
be transmitted.
A summary of the Configure-Request packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
1 for Configure-Request.
Identifier
The Identifier field SHOULD be changed on each transmission. On
reception, the Identifier field should be copied into the
Identifier field of the appropriate reply packet.
Options
The options field is variable in length and contains the list of
zero or more Configuration Options that the sender desires to
negotiate. All Configuration Options are always negotiated
simultaneously. The format of Configuration Options is further
described in a later section.
Simpson [Page 30]

RFC 1331 Point-to-Point Protocol May 19926.2. Configure-Ack
Description
If every Configuration Option received in a Configure-Request is
both recognizable and acceptable, then a LCP implementation should
transmit a LCP packet with the Code field set to 2 (Configure-
Ack), the Identifier field copied from the received Configure-
Request, and the Options field copied from the received
Configure-Request. The acknowledged Configuration Options MUST
NOT be reordered or modified in any way.
On reception of a Configure-Ack, the Identifier field must match
that of the last transmitted Configure-Request. Additionally, the
Configuration Options in a Configure-Ack must exactly match those
of the last transmitted Configure-Request. Invalid packets are
silently discarded.
A summary of the Configure-Ack packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
2 for Configure-Ack.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Ack.
Options
The Options field is variable in length and contains the list of
zero or more Configuration Options that the sender is
acknowledging. All Configuration Options are always acknowledged
simultaneously.
Simpson [Page 31]

RFC 1331 Point-to-Point Protocol May 19926.3. Configure-Nak
Description
If every element of the received Configuration Options is
recognizable but some are not acceptable, then a LCP
implementation should transmit a LCP packet with the Code field
set to 3 (Configure-Nak), the Identifier field copied from the
received Configure-Request, and the Options field filled with only
the unacceptable Configuration Options from the Configure-Request.
All acceptable Configuration Options are filtered out of the
Configure-Nak, but otherwise the Configuration Options from the
Configure-Request MUST NOT be reordered.
Each of the Nak'd Configuration Options MUST be modified to a
value acceptable to the Configure-Nak sender. Options which have
no value fields (boolean options) use the Configure-Reject reply
instead.
Finally, an implementation may be configured to request the
negotiation of a specific option. If that option is not listed,
then that option may be appended to the list of Nak'd
Configuration Options in order to request the peer to list that
option in its next Configure-Request packet. Any value fields for
the option MUST indicate values acceptable to the Configure-Nak
sender.
On reception of a Configure-Nak, the Identifier field must match
that of the last transmitted Configure-Request. Invalid packets
are silently discarded.
Reception of a valid Configure-Nak indicates that a new
Configure-Request MAY be sent with the Configuration Options
modified as specified in the Configure-Nak.
Some Configuration Options have a variable length. Since the
Nak'd Option has been modified by the peer, the implementation
MUST be able to handle an Option length which is different from
the original Configure-Request.
Simpson [Page 32]

RFC 1331 Point-to-Point Protocol May 1992
A summary of the Configure-Nak packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
3 for Configure-Nak.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Nak.
Options
The Options field is variable in length and contains the list of
zero or more Configuration Options that the sender is Nak'ing.
All Configuration Options are always Nak'd simultaneously.
6.4. Configure-Reject
Description
If some Configuration Options received in a Configure-Request are
not recognizable or are not acceptable for negotiation (as
configured by a network administrator), then a LCP implementation
should transmit a LCP packet with the Code field set to 4
(Configure-Reject), the Identifier field copied from the received
Configure-Request, and the Options field filled with only the
unacceptable Configuration Options from the Configure-Request.
All recognizable and negotiable Configuration Options are filtered
out of the Configure-Reject, but otherwise the Configuration
Options MUST NOT be reordered or modified in any way.
On reception of a Configure-Reject, the Identifier field must
match that of the last transmitted Configure-Request.
Additionally, the Configuration Options in a Configure-Reject must
be a proper subset of those in the last transmitted Configure-
Request. Invalid packets are silently discarded.
Simpson [Page 33]

RFC 1331 Point-to-Point Protocol May 1992
Reception of a valid Configure-Reject indicates that a new
Configure-Request SHOULD be sent which does not include any of the
Configuration Options listed in the Configure-Reject.
A summary of the Configure-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
4 for Configure-Reject.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Reject.
Options
The Options field is variable in length and contains the list of
zero or more Configuration Options that the sender is rejecting.
All Configuration Options are always rejected simultaneously.
Simpson [Page 34]

RFC 1331 Point-to-Point Protocol May 19926.5. Terminate-Request and Terminate-Ack
Description
LCP includes Terminate-Request and Terminate-Ack Codes in order to
provide a mechanism for closing a connection.
A LCP implementation wishing to close a connection should transmit
a LCP packet with the Code field set to 5 (Terminate-Request) and
the Data field filled with any desired data. Terminate-Request
packets should continue to be sent until Terminate-Ack is
received, the lower layer indicates that it has gone down, or a
sufficiently large number have been transmitted such that the peer
is down with reasonable certainty.
Upon reception of a Terminate-Request, a LCP packet MUST be
transmitted with the Code field set to 6 (Terminate-Ack), the
Identifier field copied from the Terminate-Request packet, and the
Data field filled with any desired data.
Reception of an unelicited Terminate-Ack indicates that the peer
is in the Closed or Stopped states, or is otherwise in need of
re-negotiation.
A summary of the Terminate-Request and Terminate-Ack packet formats
is shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
5 for Terminate-Request;
6 for Terminate-Ack.
Identifier
The Identifier field is one octet and aids in matching requests
and replies.
Simpson [Page 35]

RFC 1331 Point-to-Point Protocol May 1992
Data
The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
maximum Information field length minus four.
6.6. Code-Reject
Description
Reception of a LCP packet with an unknown Code indicates that one
of the communicating LCP implementations is faulty or incomplete.
This error MUST be reported back to the sender of the unknown Code
by transmitting a LCP packet with the Code field set to 7 (Code-
Reject), and the inducing packet copied to the Rejected-
Information field.
Upon reception of a Code-Reject, the implementation SHOULD report
the error, since it is unlikely that the situation can be
rectified automatically.
A summary of the Code-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Packet ...
+-+-+-+-+-+-+-+-+
Code
7 for Code-Reject.
Identifier
The Identifier field is one octet and is for use by the
transmitter.
Rejected-Information
The Rejected-Information field contains a copy of the LCP packet
which is being rejected. It begins with the Information field,
and does not include any PPP Data Link Layer headers nor the FCS.
Simpson [Page 36]

RFC 1331 Point-to-Point Protocol May 1992
The Rejected-Information MUST be truncated to comply with the
peer's established maximum Information field length.
Simpson [Page 37]

RFC 1331 Point-to-Point Protocol May 19926.7. Protocol-Reject
Description
Reception of a PPP frame with an unknown Data Link Layer Protocol
indicates that the peer is attempting to use a protocol which is
unsupported. This usually occurs when the peer attempts to
configure a new protocol. If the LCP state machine is in the
Opened state, then this error MUST be reported back to the peer by
transmitting a LCP packet with the Code field set to 8 (Protocol-
Reject), the Rejected-Protocol field set to the received Protocol,
and the inducing packet copied to the Rejected-Information field.
Upon reception of a Protocol-Reject, a LCP implementation SHOULD
stop transmitting frames of the indicated protocol.
Protocol-Reject packets may only be sent in the LCP Opened state.
Protocol-Reject packets received in any state other than the LCP
Opened state SHOULD be silently discarded.
A summary of the Protocol-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Protocol | Rejected-Information ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
8 for Protocol-Reject.
Identifier
The Identifier field is one octet and is for use by the
transmitter.
Rejected-Protocol
The Rejected-Protocol field is two octets and contains the
Protocol of the Data Link Layer frame which is being rejected.
Rejected-Information
The Rejected-Information field contains a copy from the frame
Simpson [Page 38]

RFC 1331 Point-to-Point Protocol May 1992
which is being rejected. It begins with the Information field,
and does not include any PPP Data Link Layer headers nor the FCS.
The Rejected-Information MUST be truncated to comply with the
peer's established maximum Information field length.
6.8. Echo-Request and Echo-Reply
Description
LCP includes Echo-Request and Echo-Reply Codes in order to provide
a Data Link Layer loopback mechanism for use in exercising both
directions of the link. This is useful as an aid in debugging,
link quality determination, performance testing, and for numerous
other functions.
An Echo-Request sender transmits a LCP packet with the Code field
set to 9 (Echo-Request), the Identifier field set, the local
Magic-Number inserted, and the Data field filled with any desired
data, up to but not exceeding the peer's established maximum
Information field length minus eight.
Upon reception of an Echo-Request, a LCP packet MUST be
transmitted with the Code field set to 10 (Echo-Reply), the
Identifier field copied from the received Echo-Request, the local
Magic-Number inserted, and the Data field copied from the Echo-
Request, truncating as necessary to avoid exceeding the peer's
established maximum Information field length.
Echo-Request and Echo-Reply packets may only be sent in the LCP
Opened state. Echo-Request and Echo-Reply packets received in any
state other than the LCP Opened state SHOULD be silently
discarded.
A summary of the Echo-Request and Echo-Reply packet formats is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic-Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Simpson [Page 39]

RFC 1331 Point-to-Point Protocol May 1992
Code
9 for Echo-Request;
10 for Echo-Reply.
Identifier
The Identifier field is one octet and aids in matching Echo-
Requests and Echo-Replies.
Magic-Number
The Magic-Number field is four octets and aids in detecting links
which are in the looped-back condition. Unless modified by a
Configuration Option, the Magic-Number MUST be transmitted as zero
and MUST be ignored on reception. See the Magic-Number
Configuration Option for further explanation.
Data
The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
maximum Information field length minus eight.
6.9. Discard-Request
Description
LCP includes a Discard-Request Code in order to provide a Data
Link Layer data sink mechanism for use in exercising the local to
remote direction of the link. This is useful as an aid in
debugging, performance testing, and for numerous other functions.
A discard sender transmits a LCP packet with the Code field set to
11 (Discard-Request) the Identifier field set, the local Magic-
Number inserted, and the Data field filled with any desired data,
up to but not exceeding the peer's established maximum Information
field length minus eight.
A discard receiver MUST simply throw away an Discard-Request that
it receives.
Discard-Request packets may only be sent in the LCP Opened state.
Simpson [Page 40]

RFC 1331 Point-to-Point Protocol May 1992
A summary of the Discard-Request packet formats is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic-Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
11 for Discard-Request.
Identifier
The Identifier field is one octet and is for use by the Discard-
Request transmitter.
Magic-Number
The Magic-Number field is four octets and aids in detecting links
which are in the looped-back condition. Unless modified by a
configuration option, the Magic-Number MUST be transmitted as zero
and MUST be ignored on reception. See the Magic-Number
Configuration Option for further explanation.
Data
The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
maximum Information field length minus four.
Simpson [Page 41]

RFC 1331 Point-to-Point Protocol May 19927. LCP Configuration Options
LCP Configuration Options allow modifications to the standard
characteristics of a point-to-point link to be negotiated.
Negotiable modifications include such things as the maximum receive
unit, async control character mapping, the link authentication
method, etc. If a Configuration Option is not included in a
Configure-Request packet, the default value for that Configuration
Option is assumed.
The end of the list of Configuration Options is indicated by the
length of the LCP packet.
Unless otherwise specified, each Configuration Option is not listed
more than once in a Configuration Options list. Some Configuration
Options MAY be listed more than once. The effect of this is
Configuration Option specific and is specified by each such
Configuration Option.
Also unless otherwise specified, all Configuration Options apply in a
half-duplex fashion. When negotiated, they apply to only one
direction of the link, typically in the receive direction when
interpreted from the point of view of the Configure-Request sender.
Simpson [Page 42]

RFC 1331 Point-to-Point Protocol May 19927.1. Format
A summary of the Configuration Option format is shown below. The
fields are transmitted from left to right.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
The Type field is one octet and indicates the type of
Configuration Option. The most up-to-date values of the LCP
Option Type field are specified in the most recent "Assigned
Numbers" RFC [11]. Current values are assigned as follows:
1 Maximum-Receive-Unit
2 Async-Control-Character-Map
3 Authentication-Protocol
4 Quality-Protocol
5 Magic-Number
6 RESERVED
7 Protocol-Field-Compression
8 Address-and-Control-Field-Compression
Length
The Length field is one octet and indicates the length of this
Configuration Option including the Type, Length and Data fields.
If a negotiable Configuration Option is received in a Configure-
Request but with an invalid Length, a Configure-Nak SHOULD be
transmitted which includes the desired Configuration Option with
an appropriate Length and Data.
Data
The Data field is zero or more octets and indicates the value or
other information for this Configuration Option. The format and
length of the Data field is determined by the Type and Length
fields.
Simpson [Page 43]

RFC 1331 Point-to-Point Protocol May 19927.3. Async-Control-Character-Map
Description
This Configuration Option provides a way to negotiate the use of
control character mapping on asynchronous links. By default, PPP
maps all control characters into an appropriate two character
sequence. However, it is rarely necessary to map all control
characters and often it is unnecessary to map any characters. A
PPP implementation may use this Configuration Option to inform the
peer which control characters must remain mapped and which control
characters need not remain mapped when the peer sends them. The
peer may still send these control characters in mapped format if
it is necessary because of constraints at the peer.
There may be some use of synchronous-to-asynchronous converters
(some built into modems) in Point-to-Point links resulting in a
synchronous PPP implementation on one end of a link and an
asynchronous implementation on the other. It is the
responsibility of the converter to do all mapping conversions
during operation. To enable this functionality, synchronous PPP
implementations MUST always accept a Async-Control-Character-Map
Configuration Option (it MUST always respond to an LCP Configure-
Request specifying this Configuration Option with an LCP
Configure-Ack). However, acceptance of this Configuration Option
does not imply that the synchronous implementation will do any
character mapping, since synchronous PPP uses bit-stuffing rather
than character-stuffing. Instead, all such character mapping will
be performed by the asynchronous-to-synchronous converter.
A summary of the Async-Control-Character-Map Configuration Option
format is shown below. The fields are transmitted from left to
right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Async-Control-Character-Map
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ACCM (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Simpson [Page 45]

RFC 1331 Point-to-Point Protocol May 1992
Length
6
Async-Control-Character-Map
The Async-Control-Character-Map field is four octets and indicates
the new async control character map. The map is encoded in big-
endian fashion where each numbered bit corresponds to the ASCII
control character of the same value. If the bit is cleared to
zero, then that ASCII control character need not be mapped. If
the bit is set to one, then that ASCII control character must
remain mapped. E.g., if bit 19 is set to zero, then the ASCII
control character 19 (DC3, Control-S) may be sent in the clear.
Note: The bit ordering of the map is as described in section3.1, Most Significant Bit to Least Significant Bit. The least
significant bit of the least significant octet (the final octet
transmitted) is numbered bit 0, and would map to the ASCII
control character NUL.
Default
All ones (0xffffffff).
Simpson [Page 46]

RFC 1331 Point-to-Point Protocol May 19927.4. Authentication-Protocol
Description
On some links it may be desirable to require a peer to
authenticate itself before allowing network-layer protocol packets
to be exchanged. This Configuration Option provides a way to
negotiate the use of a specific authentication protocol. By
default, authentication is not necessary.
An implementation SHOULD NOT include multiple Authentication-
Protocol Configuration Options in its Configure-Request packets.
Instead, it SHOULD attempt to configure the most desirable
protocol first. If that protocol is Rejected, then the
implementation could attempt the next most desirable protocol in
the next Configure-Request.
An implementation receiving a Configure-Request specifying
Authentication-Protocols MAY choose at most one of the negotiable
authentication protocols and MUST send a Configure-Reject
including the other specified authentication protocols. The
implementation MAY reject all of the proposed authentication
protocols.
If an implementation sends a Configure-Ack with this Configuration
Option, then it is agreeing to authenticate with the specified
protocol. An implementation receiving a Configure-Ack with this
Configuration Option SHOULD expect the peer to authenticate with
the acknowledged protocol.
There is no requirement that authentication be full duplex or that
the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.
A summary of the Authentication-Protocol Configuration Option format
is shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Authentication-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Simpson [Page 47]

RFC 1331 Point-to-Point Protocol May 1992
Type
3
Length
>= 4
Authentication-Protocol
The Authentication-Protocol field is two octets and indicates the
authentication protocol desired. Values for this field are always
the same as the PPP Data Link Layer Protocol field values for that
same authentication protocol.
The most up-to-date values of the Authentication-Protocol field
are specified in the most recent "Assigned Numbers" RFC [11].
Current values are assigned as follows:
Value (in hex) Protocol
c023 Password Authentication Protocol
c223 Challenge Handshake Authentication
Protocol
Data
The Data field is zero or more octets and contains additional data
as determined by the particular protocol.
Default
No authentication protocol necessary.
Simpson [Page 48]

RFC 1331 Point-to-Point Protocol May 19927.5. Quality-Protocol
Description
On some links it may be desirable to determine when, and how
often, the link is dropping data. This process is called link
quality monitoring.
This Configuration Option provides a way to negotiate the use of a
specific protocol for link quality monitoring. By default, link
quality monitoring is disabled.
There is no requirement that quality monitoring be full duplex or
that the same protocol be used in both directions. It is
perfectly acceptable for different protocols to be used in each
direction. This will, of course, depend on the specific protocols
negotiated.
A summary of the Quality-Protocol Configuration Option format is
shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Quality-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Type
4
Length
>= 4
Quality-Protocol
The Quality-Protocol field is two octets and indicates the link
quality monitoring protocol desired. Values for this field are
always the same as the PPP Data Link Layer Protocol field values
for that same monitoring protocol.
The most up-to-date values of the Quality-Protocol field are
specified in the most recent "Assigned Numbers" RFC [11]. Current
values are assigned as follows:
Simpson [Page 49]

RFC 1331 Point-to-Point Protocol May 19927.6. Magic-Number
Description
This Configuration Option provides a way to detect looped-back
links and other Data Link Layer anomalies. This Configuration
Option MAY be required by some other Configuration Options such as
the Monitoring-Protocol Configuration Option.
Before this Configuration Option is requested, an implementation
must choose its Magic-Number. It is recommended that the Magic-
Number be chosen in the most random manner possible in order to
guarantee with very high probability that an implementation will
arrive at a unique number. A good way to choose a unique random
number is to start with an unique seed. Suggested sources of
uniqueness include machine serial numbers, other network hardware
addresses, time-of-day clocks, etc. Particularly good random
number seeds are precise measurements of the inter-arrival time of
physical events such as packet reception on other connected
networks, server response time, or the typing rate of a human
user. It is also suggested that as many sources as possible be
used simultaneously.
When a Configure-Request is received with a Magic-Number
Configuration Option, the received Magic-Number is compared with
the Magic-Number of the last Configure-Request sent to the peer.
If the two Magic-Numbers are different, then the link is not
looped-back, and the Magic-Number should be acknowledged. If the
two Magic-Numbers are equal, then it is possible, but not certain,
that the link is looped-back and that this Configure-Request is
actually the one last sent. To determine this, a Configure-Nak
should be sent specifying a different Magic-Number value. A new
Configure-Request should not be sent to the peer until normal
processing would cause it to be sent (i.e., until a Configure-Nak
is received or the Restart timer runs out).
Reception of a Configure-Nak with a Magic-Number different from
that of the last Configure-Nak sent to the peer proves that a link
is not looped-back, and indicates a unique Magic-Number. If the
Magic-Number is equal to the one sent in the last Configure-Nak,
the possibility of a looped-back link is increased, and a new
Magic-Number should be chosen. In either case, a new Configure-
Request should be sent with the new Magic-Number.
If the link is indeed looped-back, this sequence (transmit
Configure-Request, receive Configure-Request, transmit Configure-
Nak, receive Configure-Nak) will repeat over and over again. If
the link is not looped-back, this sequence might occur a few
Simpson [Page 51]

RFC 1331 Point-to-Point Protocol May 1992
times, but it is extremely unlikely to occur repeatedly. More
likely, the Magic-Numbers chosen at either end will quickly
diverge, terminating the sequence. The following table shows the
probability of collisions assuming that both ends of the link
select Magic-Numbers with a perfectly uniform distribution:
Number of Collisions Probability
-------------------- ---------------------
1 1/2**32 = 2.3 E-10
2 1/2**32**2 = 5.4 E-20
3 1/2**32**3 = 1.3 E-29
Good sources of uniqueness or randomness are required for this
divergence to occur. If a good source of uniqueness cannot be
found, it is recommended that this Configuration Option not be
enabled; Configure-Requests with the option SHOULD NOT be
transmitted and any Magic-Number Configuration Options which the
peer sends SHOULD be either acknowledged or rejected. In this
case, loop-backs cannot be reliably detected by the
implementation, although they may still be detectable by the peer.
If an implementation does transmit a Configure-Request with a
Magic-Number Configuration Option, then it MUST NOT respond with a
Configure-Reject if its peer also transmits a Configure-Request
with a Magic-Number Configuration Option. That is, if an
implementation desires to use Magic Numbers, then it MUST also
allow its peer to do so. If an implementation does receive a
Configure-Reject in response to a Configure-Request, it can only
mean that the link is not looped-back, and that its peer will not
be using Magic-Numbers. In this case, an implementation should
act as if the negotiation had been successful (as if it had
instead received a Configure-Ack).
The Magic-Number also may be used to detect looped-back links
during normal operation as well as during Configuration Option
negotiation. All LCP Echo-Request, Echo-Reply, and Discard-
Request packets have a Magic-Number field which MUST normally be
zero, and MUST normally be ignored on reception. If Magic-Number
has been successfully negotiated, an implementation MUST transmit
these packets with the Magic-Number field set to its negotiated
Magic-Number.
The Magic-Number field of these packets SHOULD be inspected on
reception. All received Magic-Number fields MUST be equal to
either zero or the peer's unique Magic-Number, depending on
whether or not the peer negotiated one.
Reception of a Magic-Number field equal to the negotiated local
Simpson [Page 52]

RFC 1331 Point-to-Point Protocol May 1992
Magic-Number indicates a looped-back link. Reception of a Magic-
Number other than the negotiated local Magic-Number or the peer's
negotiated Magic-Number, or zero if the peer didn't negotiate one,
indicates a link which has been (mis)configured for communications
with a different peer.
Procedures for recovery from either case are unspecified and may
vary from implementation to implementation. A somewhat
pessimistic procedure is to assume a LCP Down event. A further
Open event will begin the process of re-establishing the link,
which can't complete until the loop-back condition is terminated
and Magic-Numbers are successfully negotiated. A more optimistic
procedure (in the case of a loop-back) is to begin transmitting
LCP Echo-Request packets until an appropriate Echo-Reply is
received, indicating a termination of the loop-back condition.
A summary of the Magic-Number Configuration Option format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Magic-Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Magic-Number (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
5
Length
6
Magic-Number
The Magic-Number field is four octets and indicates a number which
is very likely to be unique to one end of the link. A Magic-
Number of zero is illegal and MUST always be Nak'd, if it is not
Rejected outright.
Default
None.
Simpson [Page 53]

RFC 1331 Point-to-Point Protocol May 19927.7. Protocol-Field-Compression
Description
This Configuration Option provides a way to negotiate the
compression of the Data Link Layer Protocol field. By default,
all implementations MUST transmit standard PPP frames with two
octet Protocol fields. However, PPP Protocol field numbers are
chosen such that some values may be compressed into a single octet
form which is clearly distinguishable from the two octet form.
This Configuration Option is sent to inform the peer that the
implementation can receive such single octet Protocol fields.
Compressed Protocol fields MUST NOT be transmitted unless this
Configuration Option has been negotiated.
As previously mentioned, the Protocol field uses an extension
mechanism consistent with the ISO 3309 extension mechanism for the
Address field; the Least Significant Bit (LSB) of each octet is
used to indicate extension of the Protocol field. A binary "0" as
the LSB indicates that the Protocol field continues with the
following octet. The presence of a binary "1" as the LSB marks
the last octet of the Protocol field. Notice that any number of
"0" octets may be prepended to the field, and will still indicate
the same value (consider the two representations for 3, 00000011
and 00000000 00000011).
In the interest of simplicity, the standard PPP frame uses this
fact and always sends Protocol fields with a two octet
representation. Protocol field values less than 256 (decimal) are
prepended with a single zero octet even though transmission of
this, the zero and most significant octet, is unnecessary.
However, when using low speed links, it is desirable to conserve
bandwidth by sending as little redundant data as possible. The
Protocol Compression Configuration Option allows a trade-off
between implementation simplicity and bandwidth efficiency. If
successfully negotiated, the ISO 3309 extension mechanism may be
used to compress the Protocol field to one octet instead of two.
The large majority of frames are compressible since data protocols
are typically assigned with Protocol field values less than 256.
In addition, PPP implementations must continue to be robust and
MUST accept PPP frames with either double-octet or single-octet
Protocol fields, and MUST NOT distinguish between them.
The Protocol field is never compressed when sending any LCP
packet. This rule guarantees unambiguous recognition of LCP
packets.
Simpson [Page 54]

RFC 1331 Point-to-Point Protocol May 19927.8. Address-and-Control-Field-Compression
Description
This Configuration Option provides a way to negotiate the
compression of the Data Link Layer Address and Control fields. By
default, all implementations MUST transmit frames with Address and
Control fields and MUST use the hexadecimal values 0xff and 0x03
respectively. Since these fields have constant values, they are
easily compressed. This Configuration Option is sent to inform
the peer that the implementation can receive compressed Address
and Control fields.
Compressed Address and Control fields are formed by simply
omitting them. By definition the first octet of a two octet
Protocol field will never be 0xff, and the Protocol field value
0x00ff is not allowed (reserved) to avoid ambiguity.
On reception, the Address and Control fields are decompressed by
examining the first two octets. If they contain the values 0xff
and 0x03, they are assumed to be the Address and Control fields.
If not, it is assumed that the fields were compressed and were not
transmitted.
If a compressed frame is received when Address-and-Control-Field-
Compression has not been negotiated, the implementation MAY
silently discard the frame.
The Address and Control fields MUST NOT be compressed when sending
any LCP packet. This rule guarantees unambiguous recognition of
LCP packets.
When the Address and Control fields are compressed, the Data Link
Layer FCS field is calculated on the compressed frame, not the
original uncompressed frame.
A summary of the Address-and-Control-Field-Compression configuration
option format is shown below. The fields are transmitted from left
to right.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Simpson [Page 56]

RFC 1331 Point-to-Point Protocol May 1992A. Asynchronous HDLC
This appendix summarizes the modifications to ISO 3309-1979 proposed
in ISO 3309:1984/PDAD1, as applied in the Point-to-Point Protocol.
These modifications allow HDLC to be used with 8-bit asynchronous
links.
Transmission Considerations
All octets are transmitted with one start bit, eight bits of data,
and one stop bit. There is no provision in either PPP or ISO
3309:1984/PDAD1 for seven bit asynchronous links.
Flag Sequence
The Flag Sequence is a single octet and indicates the beginning or
end of a frame. The Flag Sequence consists of the binary sequence
01111110 (hexadecimal 0x7e).
Transparency
On asynchronous links, a character stuffing procedure is used.
The Control Escape octet is defined as binary 01111101
(hexadecimal 0x7d) where the bit positions are numbered 87654321
(not 76543210, BEWARE).
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control
Escape octet and octet with value less than hexadecimal 0x20 which
is flagged in the Remote Async-Control-Character-Map is replaced
by a two octet sequence consisting of the Control Escape octet and
the original octet with bit 6 complemented (i.e., exclusive-or'd
with hexadecimal 0x20).
Prior to FCS computation, the receiver examines the entire frame
between the two Flag Sequences. Each octet with value less than
hexadecimal 0x20 is checked. If it is flagged in the Local
Async-Control-Character-Map, it is simply removed (it may have
been inserted by intervening data communications equipment). For
each Control Escape octet, that octet is also removed, but bit 6
of the following octet is complemented. A Control Escape octet
immediately preceding the closing Flag Sequence indicates an
invalid frame.
Note: The inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [10] excluding DEL (Delete)
to be transparently communicated through almost all known data
communications equipment.
Simpson [Page 58]

RFC 1331 Point-to-Point Protocol May 1992
The transmitter may also send octets with value in the range 0x40
through 0xff (except 0x5e) in Control Escape format. Since these
octet values are not negotiable, this does not solve the problem
of receivers which cannot handle all non-control characters.
Also, since the technique does not affect the 8th bit, this does
not solve problems for communications links that can send only 7-
bit characters.
A few examples may make this more clear. Packet data is
transmitted on the link as follows:
0x7e is encoded as 0x7d, 0x5e.
0x7d is encoded as 0x7d, 0x5d.
0x01 is encoded as 0x7d, 0x21.
Some modems with software flow control may intercept outgoing DC1
and DC3 ignoring the 8th (parity) bit. This data would be
transmitted on the link as follows:
0x11 is encoded as 0x7d, 0x31.
0x13 is encoded as 0x7d, 0x33.
0x91 is encoded as 0x7d, 0xb1.
0x93 is encoded as 0x7d, 0xb3.
Aborting a Transmission
On asynchronous links, frames may be aborted by transmitting a "0"
stop bit where a "1" bit is expected (framing error) or by
transmitting a Control Escape octet followed immediately by a
closing Flag Sequence.
Time Fill
On asynchronous links, inter-octet and inter-frame time fill MUST
be accomplished by transmitting continuous "1" bits (mark-hold
state).
Note: On asynchronous links, inter-frame time fill can be
viewed as extended inter-octet time fill. Doing so can save
one octet for every frame, decreasing delay and increasing
bandwidth. This is possible since a Flag Sequence may serve as
both a frame close and a frame begin. After having received
any frame, an idle receiver will always be in a frame begin
state.
Robust transmitters should avoid using this trick over-
zealously since the price for decreased delay is decreased
reliability. Noisy links may cause the receiver to receive
Simpson [Page 59]

RFC 1331 Point-to-Point Protocol May 1992
garbage characters and interpret them as part of an incoming
frame. If the transmitter does not transmit a new opening Flag
Sequence before sending the next frame, then that frame will be
appended to the noise characters causing an invalid frame (with
high reliability). Transmitters should avoid this by
transmitting an open Flag Sequence whenever "appreciable time"
has elapsed since the prior closing Flag Sequence. It is
suggested that implementations will achieve the best results by
always sending an opening Flag Sequence if the new frame is not
back-to-back with the last. The maximum value for "appreciable
time" is likely to be no greater than the typing rate of a slow
to average typist, say 1 second.
Simpson [Page 60]

RFC 1331 Point-to-Point Protocol May 1992C. LCP Recommended Options
The following Configurations Options are recommended:
SYNC LINES
Magic Number
Link Quality Monitoring
No Address and Control Field Compression
No Protocol Field Compression
ASYNC LINES
Async Control Character Map
Magic Number
Address and Control Field Compression
Protocol Field Compression
Simpson [Page 64]